APPENDIX R

STRAW-BALE STRUCTURES

SECTION AR101PURPOSE

AR101.1 Minimum standards. The purpose of this appendixchapter is to establish minimum prescriptive standards ofsafety for the construction of structures that use baled straw as aload-bearing or nonload-bearing material.

SECTION AR102SCOPE

AR102.1 Applicability. The provisions of this chapter shallapply to single family detached structures and relate accessorystructures as defined in Section R101.9, utilizing straw bales inthe construction of wall systems.

SECTION AR103DEFINITIONS

AR103.1 General. For the purpose of this chapter, certainterms are defined as follows:

BALES. Rectangular compressed blocks of straw, bound bystrings or wire.

FLAKES. Slabs of straw removed from an untied bale. Flakesare used to fill small gaps between the ends of stacked bales.

LAID FLAT. Refers to stacking bales so that the sides with thelargest cross-sectional area are horizontal and the longest di-mension of this area is parallel with the wall plane.

LAID ON EDGE. Refers to stacking bales so that the sideswith the largest cross-sectional area are vertical and the longestdimension of this area is horizontal and parallel with the wallplane.

STRAW. The dry stems of cereal grains left after the seedheads have been removed.

LOAD BEARING. The use of straw bale walls as structuralload-bearing walls supporting the roof and resisting lateralloads.

NONLOAD BEARING. A structure where straw bales areused to in-fill between the supporting structural members. Theexterior straw bale in-fill wall sections collect lateral loads re-sulting from wind or seismic activity and transfer these loads tothe primary structure for resistance.

IN-FILL WALL. The nonbearing section between verticalsupports.

SECTION AR104MATERIAL SPECIFICATIONS

AR104.1 Type of straw. Bales of various types of straw, in-cluding, but not limited to wheat, rice, rye, barley, oats and sim-

ilar plants, shall be acceptable if they meet the minimum re-quirements for density, shape, moisture content and ties.

AR104.2 Shape. Bales shall be rectangular in shape.

AR104.2.1 Dimensions. Bales used within a continuouswall shall be of consistent height and width to ensure evendistribution of loads within wall systems.

AR104.2.2 Wall thickness. Nominal minimum bale wallthickness shall be 14 inches (356 mm).

AR104.2.3 Custom size bales. Where custom made partialbales are used, they shall be of the same density, same stringor wire tension, and where possible, use the same number ofties as the standard size bales.

AR104.3 Ties. Bales shall be bound with ties of either poly-propylene string or baling wire. Bales with broken or loose tiesshall not be used unless the broken or loose ties are replacedwith ties which restore the original degree of compaction of thebale.

AR104.4 Moisture content. Moisture content of bales, at timeof installation and just prior to applying exterior weather pro-tection such as stucco, shall not exceed 20 percent of the totalweight of the bale. Bales that become wet or exceed 20-percentmoisture content shall be replaced with dry bales or allowed todry and be retested. Bales that exceed 20-percent moisture con-tent shall be replaced. Special inspection of the moisture con-tent of bales shall be performed using one of he followingmethods:

AR104.4.1 Field method. A suitable moisture meter, de-signed for use with baled straw or hay, and equipped with aprobe of sufficient length to reach the center of the bale,shall be used to determine the average moisture content offive bales randomly selected from the bales to be used.

AR104.4.2 Laboratory method. A total of five samples,taken from the center of each of five bales randomly se-lected from the bales to be used, shall be tested for moisturecontent by a recognized testing lab.

AR104.5 Density. Bales in load-bearing structures shall have aminimum calculated dry density of 7.0 pounds per cubic foot(112 kg/m3). The calculated dry density shall be determined af-ter reducing the actual bale weight by the weight of the mois-ture content, as determined in Section AR104.4. Thecalculated dry density shall be determined by dividing the cal-culated dry weight of the bale by the volume of the bale.

SECTION AR105FOUNDATION REQUIREMENTS

AR105.1 Foundations. Foundations shall comply with Chap-ter 4. Foundations shall be sized to accommodate the thicknessof the bale wall and support the imposed live and dead loadsfrom walls and roofs. The minimum width of the footing shall

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be the width of the bale it supports, except that the bales mayoverhang the exterior edge of the foundation by not more than 3inches (76 mm) to accommodate rigid perimeter insulation.

AR105.2 Wall reinforcement embedded in foundation. Ver-tical reinforcing bars with a minimum diameter of 1/2 inch (12.7mm), shall be embedded in the foundation a minimum depth of6 inches (153 mm), and shall extend above the foundation aminimum of 12 inches (304.8 mm). These vertical bars shall belocated along the centerline of the bale wall, spaced not morethan 2 feet (610 mm) apart. A vertical bar shall also be locatedwithin 1 foot (305 mm) of any opening or corner, except at lo-cations occupied by anchor bolts.

AR105.3 Moisture barrier. A moisture barrier shall be usedbetween the top of the foundation and the bottom of the balewall to prevent moisture from migrating through the founda-tion into the bottom course of bales. This barrier shall consist ofone of the following.

1. Cementious waterproof coating;

2. Type 30 asphalt felt over an asphalt emulsion;

3. Sheet metal flashing, sealed at joints; or

4. Other approved building moisture barrier.

All penetrations through the moisture barrier, as well as alljoints in the barrier, must be sealed with asphalt, caulking or anapproved sealant.

SECTION AR106WALL CONSTRUCTION

AR106.1 General (fire resistance). Bale walls, when coveredwith plaster or stucco shall be deemed to be fire resistive.

A straw bale covered with plaster or stucco having a mini-mum thickness of 7/8 inch (22.4 mm) on both sides maybe usedbetween a dwelling and attached garage as required in SectionR309.2.

AR106.2 Allowable Loads. The allowable vertical load (liveand dead load) on the top of load-bearing bale walls shall notexceed 360 pounds per square foot (psf) (17.2 kPa) [23 inches(84.2 mm) divided by 12 inches (304.8 mm) by 360 pounds(17.2 kPa) = 690 pounds per linear foot (plf) (10.1 N/mm] andthe resultant load shall act at the center of the wall. Bale struc-tures shall be designed to withstand all vertical and horizontalloads as specified in Chapter 3 and the Structural Code, Chap-ter 16. Increases in stress and reductions of live loads specifiedin Structural Code Chapter 16 may be applied.

AR106.3 Insulation. Exterior walls using straw bales shall beconsidered to meet the wall insulation requirements of Path 1as described in Section N1104. All other exterior wall compo-nents shall comply with Section N1104. For the purpose of cal-culating thermal performance using Table N1104.1.2, theinsulation of straw bale walls can assume to be R-2.1 per inch[a 23-inch (584 mm) bale would be R-48 or U-0.021].

AR106.4 Intersecting walls. Walls of other materials inter-secting bale walls shall be attached to the bale wall by means ofone or more of the following methods or an acceptable equiva-lent:

1. Wooden dowels at least 5/8 inch (16 mm) in diameter, ofsufficient length to provide 12 inches (305 mm) of pene-tration into the bale, driven through holes bored in theabutting wall stud, and spaced to provide one dowel con-nection per bale.

2. Pointed wooden stakes, at least 12 inches (305 mm) inlength and 11/2 inch by 31/2 (38.1 mm by 88.9 mm) at theexposed end, fully driven into each course of bales, as an-chorage points.

3. Bolted or threaded rod connection of the abutting wall,through the bale wall, to a steel nut and steel or plywoodplate washer, a minimum of 6 inches (153 mm) squareand a minimum thickness of 3/16 inch (4.8 mm) for steeland 1/2 inch (12.7 mm) for plywood, in at least three loca-tions.

SECTION AR107LOAD-BEARING WALLS

AR107.1 Anchor bolts. In addition to vertical reinforcementbars required in Section M105.2, load-bearing bale walls shallbe anchored to the foundation by 1/2-inch (12.7 mm) diametersteel anchor bolts embedded at least 7 inches (178 mm) in thefoundation at intervals of 6 feet (1829 mm) or less. A minimumof two anchor bolts per wall shall be provided with one bolt lo-cated within 36 inches (914 mm) of each end of each wall. Sec-tions of 1/2-inch (12.7 mm) diameter threaded rod shall beconnected to the anchor bolts, and to each other, by means ofthreaded coupling nuts and shall extend through the roof-bear-ing assembly and be fastened with a steel washer and nut. Balewalls and roof-bearing assemblies may be anchored to thefoundation by means of other methods, which are adequate toresist uplift forces resulting from the design wind load. Thereshall be a minimum of two points of anchorage per wall, spacednot more than 6 feet (1829 mm) apart, with one located within36 inches (914 mm) of each end of each wall.

The dead load of the roof and ceiling systems will producevertical compression of the bales. Regardless of the anchoringsystem used to attach the roof bearing assembly to the founda-tion, prior to installation of wall finish materials, bolts, straps orcables shall be retightened to compensate for this compressionafter the actual dead load of the roof framing and roofing havebeen applied.

AR107.2 Load-bearing wall height. Load-bearing straw balewalls shall not exceed one story in height and the bale portionshall not exceed a height to width ratio of 5.6:1 [for example,the maximum height for the bale portion of a 23-inch-thick(584 mm) wall would be 10 feet, 8 inches (3251 mm)].

AR107.3 Unsupported wall length. The ratio of unsupportedwall length to thickness, for load-bearing bale walls, shall notexceed 13:1 [for a 23-inch (584 mm) thick wall, the maximumunsupported length allowed is 25 feet (7620 mm)].

AR107.4 Openings and lintels. All openings in load-bearingbale walls shall be a minimum of one full bale length from anyoutside corner.

AR107.4.1 Openings. Openings in exterior bale walls shallnot exceed 50 percent of the total wall area, based on interior

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dimensions, where the wall is providing resistance to lateralloads.

Door and window frames (rough bucks) shall be stabi-lized with 1/2-inch (12.7 mm) diameter hardwood dowelsextending 12 inches (305 mm) into every adjacent bale or bymeans of a continuous metal lath applied to both the interiorand exterior, extending a minimum of 12 inches (305 mm)onto adjacent bales, installed prior to the application of plas-ter or stucco.

AR107.4.2 Lintels. Wall and/or roof load present above anyopening shall be carried, or transferred to the bales below byone of the following:

1. A structural frame,

2. A lintel (such as an angle-iron cradle, wooden beam,wooden box beam). Lintels shall be at least twice aslong as the opening is wide and extend at least 24inches (610 mm) beyond either side of the opening.Lintels shall be centered over openings, and shall notexceed the load limitations of Section AR106.2 bymore than 25 percent.

3. A roof-bearing assembly designed to act as a rigidbeam over the opening.

AR107.5 Stacking. Bales in load-bearing walls shall be laidflat and stacked in running bond where possible, with each baleoverlapping the two bales beneath it. Bales in nonload-bearingwalls may be laid either flat or on-edge and stacked in runningbond where possible. Overlaps shall be a minimum of 12inches (305 mm). Gaps between the ends of bales which areless than 6 inches (153 mm) in width may be filled by an untiedflake inserted snugly into the gap. Only full-length bales shallbe used at corners of load-bearing walls.

AR107.5.1 Pinning. The first course of bales shall be laidby impaling the bales on the vertical bars or threaded rods, ifany, extending from the foundation. When the fourth coursehas been laid, No. 4 rebar pins, or an acceptable equivalent,long enough to extend through all four courses, shall bedriven down through the bales, two in each bale, located sothat they do not pass within 6 inches (153 mm) of, orthrough the space between the ends of any two bales. Thelayout of these pins shall approximate the layout of the verti-cal rebars extending from the foundation. As each subse-quent course is laid, two such pins, long enough to extendthrough the course being laid and the three courses immedi-ately below it, shall be driven down through each bale. Thispinning method shall be continued to the top of the wall. Inwalls seven or eight courses high, pinning at the fifth coursemay be eliminated.

AR107.5.1.1 Corners and openings. Vertical No. 4rebar pins, or an acceptable alternative, shall be locatedwithin 1 foot (305 mm) of all corners or door openings.

AR107.5.1.2 Staples at corners. Staples made of No. 3or larger rebar formed into a “U” shape, at least 18 inches(457 mm) long with two 6-inch (153 mm) legs, shall beused at all corners of every course, driven with one leginto the top of each abutting corner bale.

AR107.6 Gable end walls. In load bearing exterior walls withgable or shed roofs, a continuous assembly as described in Sec-

tion AR110.1 shall be provided that continues through thegable end walls.

SECTION AR108NONLOAD-BEARING WALLS

AR108.1 General. A nonload-bearing wall is part of a struc-tural system where the primary frame is an approved systemcomplying with this code or Structural Code Chapter 16. Thestraw bales are used as in-fill between structural members orframing.

AR108.2 Primary frame. The primary frame (such as postand beam or pole building) shall comply with this code orStructural Code Chapter 16. Structural calculations showingcompliance shall be submitted with plans and application forbuilding permits.

AR108.2.1 Anchorage to foundation. The primary frameshall be anchored to the foundation in accordance withChapters 4 and 6 of this code or Structural Code Chapter 16.

AR108.3 Wall anchorage to foundation. Vertical reinforcingbars with a minimum diameter of 1/2 inch (12.7 mm) shall beembedded in the foundation a minimum depth of 6 inches (153mm), and shall extend above the foundation a minimum ofone-and-one-half bales.

AR108.3.1 Location. The vertical bars shall be locatedalong the centerline of the bale wall, spaced not more than 2feet (610 mm) apart and a minimum of two per bale.

AR108.3.2 Corners and openings. A vertical bar shall belocated within 1 foot (305 mm) of any opening or corner, ex-cept at locations occupied by anchor bolts.

AR108.4 Wall connections to primary frame. The straw balein-fill walls shall be securely anchored to all adjacent structuralmembers to sufficiently resist lateral displacement of the strawbale in-fill walls.

AR108.4.1 Anchors. Anchors shall be placed at every hori-zontal joint or one per bale along the vertical member and amaximum of 24 inches (610 mm) on the center along hori-zontal members at the top of the straw bale wall beginningnot more than 12 inches (305 mm) from each end of thein-fill.

AR108.4.2 Metal lath. At all points where the straw balesare butted against a different material (wood, concrete,steel, etc.), metal lath shall be used to cover the junction. Ex-panded metal lath shall extend a minimum of 6 inches (153mm) over the edge of the straw bales and shall be securelyfastened to the straw bales.

AR108.5 Stacking and pinning. Nonbearing straw bale wallsshall be stacked and pinned as described in Section AR107.5

AR108.6 Openings and lintels. The limitations of the open-ings relative to wall area is controlled by the design of the pri-mary frame.

AR108.7 Wall Height. Nonload-bearing walls shall not ex-ceed 12 feet (3658 mm) in height.

AR108.7.1 Gable end walls. In the nonload-bearing exte-rior wall with gable or shed roofs, a continuous assembly asdescribed in Section AR110.1 shall be provided that contin-

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ues through the gable end wall or shall meet the require-ments of Section R601.2.

SECTION AR109WALL FINISHES

AR109.1 Moisture protection. All weather-exposed balewalls shall be protected from water damage. An approvedbuilding moisture barrier shall be used to protect at least thebottom course of bales, but not more than the lower one-thirdof the vertical exterior wall surface, in order to allow naturaltranspiration of moisture from the bales. The moisture barriershall have its upper edge inserted at least 6 inches (153 mm)into the horizontal joint between two courses of bales, and shallextend at least 3 inches (76 mm) below the top of the founda-tion. Bale walls shall have special moisture protection providedat all window sills. Unless protected by a roof, the tops of wallsshall also be protected. This moisture protection shall consistof a waterproof membrane, such as asphalt-impregnated feltpaper, polyethylene sheeting, or other acceptable moisture bar-rier, installed in such manner as to prevent water from enteringthe wall system at window sills or at the tops of walls.

A vapor barrier paint having a 1-perm dry cup rating or lessshall be applied to the interior wall finish of straw bale wallswhich separate heated and nonheated spaces and exterior wallsfor compliance with Section N1104.9.1.

AR109.2 Protection. Interior and exterior surfaces of balewalls shall be protected from mechanical damage, flames, ani-mals, and prolonged exposure to water or snow. Bale walls ad-jacent to bath and shower enclosures shall be protected by amoisture barrier.

AR109.2.1 Stucco. Cement stucco shall be reinforced withgalvanized woven wire, stucco netting or an acceptableequivalent. Such reinforcement shall be secured by attach-ment through the wall at a maximum spacing of 24 inches(610 mm) horizontally and 16 inches (406 mm) vertically,using a method approved by the building official.

Where bales abut other materials the plaster/stucco shallbe reinforced with galvanized expanded metal lath, or an ac-ceptable equivalent, extending a minimum of 6 inches (153mm) onto the bales.

AR109.2.2 Plasters. Earthen and lime-based plasters maybe applied directly onto the exterior and interior surface ofbale walls without reinforcement, except where appliedover materials other than straw. Weather-exposed earthenplasters shall be Portland cement stabilized using methodsof application approved by the building official.

Lime based plasters may be applied directly onto the ex-terior surface of bale walls without reinforcement, exceptwhere applied over materials other than straw.

SECTION AR110ROOF-TO-WALL CONNECTION

AR110.1 Roof-bearing assembly. Load-bearing bale wallsshall have a roof-bearing assembly at the top of the wall to bearthe roof load and to provide a means of connecting the roof

structure to the foundation. The roof-bearing assembly shall becontinuous along the tops of load bearing bale walls.

AR110.1.1 Assembly option. An acceptable roof-bearingassembly option consists of two double 2 × 6, or larger, hori-zontal top plates, one located at the inner edge of the walland the other at the outer edge. Connecting the two doubledtop plates and located horizontally and perpendicular to thelength of the wall shall be 2 × 6 cross members spaced nomore than 72 inches (1829 mm) center to center, and as re-quired to align with the threaded rods extending from the an-chor bolts in the foundation. The double 2 × 6 top platesshall be face nailed with 16d nails staggered at 16 inches(406 mm) on center, with laps and intersections face nailedwith four 16d nails. The cross members shall be face nailedto the top plates with four 16d nails at each end. Corner con-nections shall include overlaps nailed as above or an accept-able equivalent such as plywood gussets or metal plates.Alternatives to this roof-bearing assembly option shall pro-vide equal or greater vertical rigidity and provide horizontalrigidity equivalent to a continuous double 2 × 4 top plate.

The connection of roof framing members to the roof bear-ing assembly shall comply with the appropriate sections ofthis code.

SECTION AR111ELECTRICAL, PLUMBING AND MECHANICAL

AR111.1 Electrical. All wiring within or on bale walls shallmeet the electrical provisions in this code. Type NM or UF4 ca-ble may be used, or wiring may be run in metallic or nonmetal-lic conduit systems.

Electrical boxes shall be securely attached to wooden stakesdriven a minimum of 12 inches (305 mm) into the bales, or anacceptable equivalent.

AR111.2 Plumbing. Water or DWV pipes within bale wallsshall be encased in a continuous pipe sleeve to prevent leakagewithin the wall. Where pipes are mounted on bale walls, theyshall be isolated from the bales by a moisture barrier.

AR111.3 Mechanical. Gas pipes within bale walls shall be en-cased in a continuous pipe sleeve to prevent leakage within thewall. Where pipes are mounted on bale walls, they shall be iso-lated from the bales by a moisture barrier.

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